Insights and Implications of Intricate Surface Charge Transfer and sp3-Defects in Graphene/Metal Oxide Interfaces

Adherence of metal oxides to graphene is of fundamental significance to graphene nanoelectronic and spintronic interfaces. Titanium oxide and aluminum oxide are two widely used tunnel barriers in such devices, which offer optimum interface resistance and distinct interface conditions that govern transport parameters and device performance. Here, we reveal a fundamental difference in how these metal oxides interface with graphene through electrical transport measurements and Raman and photoelectron spectroscopies, combined with ab initio electronic structure calculations of such interfaces. While both oxide layers cause surface charge transfer induced p-type doping in graphene, in sharp contrast to TiOx, the AlOx/graphene interface shows the presence of appreciable sp3 defects. Electronic structure calculations disclose that significant p-type doping occurs due to a combination of sp3 bonds formed between C and O atoms at the interface and possible slightly off-stoichiometric defects of the aluminum oxide layer. Furthermore, the sp3 hybridization at the AlOx/graphene interface leads to distinct magnetic moments of unsaturated bonds, which not only explicates the widely observed low spin-lifetimes in AlOx barrier graphene spintronic devices but also suggests possibilities for new hybrid resistive switching and spin valves.


Sample characterization 2a. Electrical Measurements
Electrical transport measurements were performed in high-vacuum conditions (∼10 -7 mbar) with a room-temperature setup, using a Keithley current source and a nanovoltmeter. In addition, gate voltage was applied by a Keithley source meter. Using the equation ( ) = | | + , where C = 5 × 10 15 V -1 s -1 , n is the carrier density, e is the electronic charge and is the residual conductivity, we estimated the charge impurity density 1 (Fig. S1a shows a representative fitting for a sample before and after the realization of AlO x ) with a variation ∆ ~5-20 × 10 11 cm -2 for both oxides, which is up to an order higher than the sp 3 defect density that we found using Raman spectroscopy. In addition, we also found low trap density ~10 11 cm -2 (trapped carriers trap = ∆ DP g /2 in the charge trapping effect 2 , with a change in Dirac point ∆ DP in hysteresis of Dirac curves, gate capacitance g , and electronic charge e) for all samples, compared to actual shift in the Dirac point due to oxide layers.  Figure S1. (a) Charge impurity density estimated by fitting the Dirac curves using ( ). Conductance change vs. gate voltage at 1 V/s gate sweep rate for graphene devices before and after oxide realization for (b) AlO x and (c) TiO x layers on graphene.

2b. Raman Spectroscopy
Raman spectroscopy was conducted with a Renishaw Reflex (Invia) Raman spectrometer for the single-layer CVD graphene (pristine graphene) and after Ti or Al deposition to see the evolution of the 2D and G peaks and the appearance of the D peak to calculate the defect concentration. To estimate the defect density via Raman spectroscopy, we used the empirical formula: with the laser line wavelength) which is applicable in high-density regimes ( ≈ 60%) 3 .

2c. Atomic Force Microscopy
Conventional AFM images of the graphene with deposited Ti and Al were obtained by Scanning Probe Microscope Bruker Dimension Icon with high resolution and low noise level for topography analysis using scan areas of 1 μm × 1 μm and 250 nm × 250 nm with corresponding height profiles and root mean square roughness. While imaging, we used PeakForce in the ScanAsyst imaging mode introduced by Bruker. The PeakForce method combines tapping mode as well as contact mode where direct force is controlled, and damaging lateral forces are avoided. The scan ScanAsyst algorithm optimizes the speed of the scan, z-position, and forces. In our samples, it was the optimum mode for high-resolution images of graphene with deposited ultra-thin metal oxides.
2d. X-ray Photoelectron Spectroscopy X-ray photoelectron spectroscopy (XPS) was performed using a Physical Electronics Quantera II Scanning XPS Microprobe with a monochromatic Al Kα source (1486.7 eV). Overview spectra were collected with a beam diameter set to 200 μm and pass energy to 224 eV, resulting in an energy resolution of 1.3 eV. For single region spectra of O 1s, Al 2p, C 1s, and Ti 2p, a 100 μm beam diameter and 55 eV pass energy was used, resulting in an energy resolution of 0.7 eV. To avoid the possibility of defect contributions from the edges in the devices, we performed XPS measurements on a 0.7 × 0.7 cm 2 Si/SiO 2 substrate fully covered with CVD graphene and top metal-oxides. XPS spectra were analyzed by correcting the background with the Shirley method and fitting core-electron peaks with Voigt functions.

Computational details for aluminum oxide simulation
The ab-initio calculations were performed using the Vienna Ab-initio Simulation Package (VASP). For all simulations, we considered the projector augmented wave (PAW) approach and the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) functional. With an energy cutoff of 520 eV, the plane-wave basis set for the AlO x /graphene structure was chosen. The ionic forces were optimized with a Monkhorst-Pack mesh of k-points of 9 × 9 × 1 subdivisions, and the convergence criterion was 0.001 eV/Å. The vacuum space in the z-periodic direction was approximately 35 Å for all investigated configurations, which is large enough to avoid interactions between periodic images. The Van der Waals (vdW) interaction was considered for the combined graphene-oxide structures using the DFT-D3 approach with Becke-Johnson damping 4 . To analyze the electronic properties of the studied systems, we combined the vasp post-processing tools 5,6 .

3a. Aluminum oxide interface construction
In this work, we considered a 2 × 2 × 1 hexagonal graphene supercell with a 1 = a 2 ~4.92 Å and a 1 × 1 × 1 aluminum oxide Al 2 O 3 unit cell with lattice vectors b 1 = b 2 ~4.76 Å. To study the interface between graphene and aluminum oxide (-Al 2 O 3 ), we compensated for the existing lattice mismatch by applying a strain of 3.25% in the Al 2 O 3 unit cell. Within this choice, we ensure that all graphene electrical-related properties are primarily preserved. In addition, an approximately 13 Å thick slab of Al 2 O 3 (correspondent to the Al 12 O 18 stoichiometry) is attached to the top of the graphene supercell in order to mimic the experimentally synthesized samples. For the simulation purpose, an initial spacing of 2 Å between these two crystals was considered.
The Al 12 O 18 slab can have (two) Al or (three) oxygen atoms near graphene. Since one can find similar results even for different geometries, we consider the amount of dangling bonds at the top and at the bottom surface of the slab as a criterion to select the number of various interfaces to be studied. In particular, we chose three interfaces that are illustrated in Fig. S2a and S2c. Additionally, we investigated the structural and electrical properties of slightly off-stoichiometric oxides (Fig. S3) when one Al or O atom next to the graphene layer is removed. Fig. S2 shows the structure before and after the atomic relaxation, as well as the band structures of all considered graphene-Al 12 O 18 systems, and summarizes the properties found for the combined structure graphene/Al 12 O 18 . In Fig. S2a, we observe that when aluminum is in proximity to the graphene layer, a flat bond between Al and O is formed, and the equilibrium distance between the two crystals exhibits an increase of 0.9 Å after force minimization. This suggests that this Al 12 O 18 geometry has a weak interaction with graphene. Consequently, no significant change at the Dirac cone of the graphene structure is observed (see Fig. S2b). However, when oxygen is in proximity to the graphene layer, two (out of three) oxygen atoms form sp 3 bonds with the carbon atoms (Fig. S2c). The unbonded oxygen holds unpaired electrons. For this reason, this oxygen shows a total magnetic moment of 0.32 µ B while the whole system has a magnetization of 0.61 µ B per unit cell.

3c. Small concentration of sp 3 bond in perfect stoichiometric oxide
To better understand the influence of the sp 3 bond in the electronic structure of pristine graphene, we created an interface with only one sp 3 bond per unit cell (rather than two, as shown in Fig. S2c). This interface is constructed by restricting the optimized system represented in Fig. S2c to just forming one sp 3 bond while preventing it from minimizing forces again. The resulting electronic band structure (see Fig. 5 in the main paper) has a significant gap widening of the Dirac cone. Furthermore, the magnetization of the entire system is lowered from 0.65 µ B to 0.5 µ B per unit cell.

3d. Slightly off-stoichiometric AlOx/graphene interface
For the slightly off-stoichiometric system, we focused on the geometry where oxygen is in proximity to the graphene layer. The reason for such analysis is that the charge transfer is expected mainly from the carbon to the oxygen atoms. For the graphene-Al 11 O 18 structure, we observe after force minimization that two sp 3 bonds are formed between the oxygen and carbon atoms (see Fig. S3a). However, in the case of graphene-Al 12 O 17, the crystals exhibit an increase of 0.9 Å of the initial distance, and no deformation in the graphene layer is observed. The total magnetization corresponding to the graphene-Al 11 O 18 structure is 1.0 µ B , while for Al 12 O 17 is 0.44 µ B . A gap opening at the Dirac cone is observed for both systems (see Figs. S3b and S3d). Furthermore, in both the band structure and spin-polarized DOS, the magnetism around the oxygen atoms near to graphene layer causes the states around the Fermi level to have different spin polarities (see Fig. S3d). Despite this, we found that Al 12 O 17 induces p-type doping in the graphene structure, whereas it opens a gap of approximately 1 eV.